The invention relates to an improved process for reducing the total sulfur content of Claus off-gases.
In a typical Claus process elemental sulfur is manufactured from hydrogen sulfide by partial oxidation of the hydrogen sulfide to sulfur dioxide with oxygen or an oxygen-containing gas such as air, followed by reaction of the sulfur dioxide formed with the remaining part of the hydrogen sulfide in the presence of a catalyst. This process is commonly used both at refineries and for working-up hydrogen sulfide recovered from natural gas. It is generally carried out in a plant comprising a combustion chamber followed by one or more catalyst beds having condensers arranged in between in which the reaction products are cooled and the separated liquid sulfur recovered. The various steps of the process can be represented by the following equations: EQU 2H.sub.2 S + 30.sub.2 .fwdarw. 2H.sub.2 O + 2SO.sub.2 ( 1) ##EQU1## WHILE THE TOTAL REACTION IS REPRESENTED BY EQUATION (3): ##EQU2## For temperatures below 500.degree. C, x in the above equation has a value of 8.
In actual practice, the yield of recovered elemental sulfur is not completely quantitative resulting in a certain quantity of unreacted hydrogen sulfide and sulfur dioxide remaining in the effluent gases from the Claus process. These gases which emanate from the Claus process at temperatures typically in the 150.degree. C range are normally burned in an incinerator whereby the hydrogen sulfide is converted to sulfur dioxide which is subsequently discharged to the atmosphere through a stack. The quantity of sulfur recovered depends to a large extent on the total number of catalyst beds used in the Claus process. When three beds are used generally about 98% of the sulfur can be recovered.
Because of increasingly stringent limitations on sulfur emissions to the atmosphere, and to increase sulfur yields, a considerable amount of effort has been devoted recently to reducing the sulfur content of Claus plant off-gases. Among the more desirable processes developed for this purpose are those based on the catalytic reduction of the sulfur oxides contained in the off-gases to hydrogen sulfide which is subsequently removed with the use of a solid adsorbent or liquid absorbent for hydrogen sulfide. Generally, the reduction of the off-gases is effected by mixing the gases with a hydrogen and/or carbon monoxide-containing reducing gas in the presence of a metal catalyst at elevated temperatures, e.g., above 175.degree. C. Such catalytic hydrogenation processes are described, for example, in co-assigned U.S. application Ser. No. 326,916 filed Jan. 26, 1973 and in U.S. Pat. No. 3,752,877 to Beavon. After catalytic reduction according to the referenced processes, the hydrogen sulfide-containing gaseous product of reduction is typically subject to a combination of cooling by indirect heat exchange and direct cooling by contact with an aqueous based quench liquid prior to removal and recovery of the H.sub.2 S as sulfur, e.g., Stretford process, or directly as H.sub.2 S, e.g., selective alkanolamine adsorption.
While processes based on catalytic reduction of Claus off-gases and removal of the H.sub.2 S so formed have achieved a large degree of commercial success such processes are not devoid of problems. For instance, problems may arise if complete reduction of sulfur compounds present in the Claus off-gas does not take place and if, for example, SO.sub.2 is still present therein which through reaction with H.sub.2 S may give rise to the formation of sulfur at undesirable locations which, in turn, may interfere with the conversion process of H.sub.2 S to sulfur, for example, in that it reacts with the absorption and/or reaction liquids to be used. Incomplete reduction of the sulfur compounds may, for example, occur if less than the required amount of reducing gas is added for the catalytic reduction. This may be the result of an interruption in the supply and/or production of the reducing gas in question.
Further the quantities of sulfur compounds, in particular SO.sub.2, in Claus off-gases may vary during operation, for example as the result of too large an air supply to the combustion chamber of the Claus plant, or of aging of the catalysts in the Claus catalyst beds. This change in Claus off-gas composition in the direction of higher SO.sub.2 concentrations without a compensating increase in the quantity of reducing gas employed could also result in incomplete reduction and undesirable carry over of SO.sub.2 into the absorption stage of the aforementioned processes.
In recognition of this potential problem source, it has been proposed in Netherlands patent application No. 7,310,929 to indirectly measure the SO.sub.2 which may carry through the reduction step and adjust the reducing gas supply accordingly, by measuring the amounts of hydrogen present in the reduced gas before or after cooling using conductometric methods. An alternative method of determining SO.sub.2 carry-through indirectly is also described in this Netherlands patent application which involves measurement of the amount of elemental sulfur (formed by the reaction of excess SO.sub.2 and H.sub.2 S) in the aqueous quench liquid used for direct cooling. With this alternative method the sulfur content is determined by measuring the turbidity of the sulfur containing quench liquid. However, either or both of the above-mentioned methods for determining SO.sub.2 breakthrough from the catalytic reduction step suffer from the disadvantages that the SO.sub.2 measurement is indirect, somewhat complex analytical equipment and/or techniques are involved and the time required to obtain meaningful values on the amounts of SO.sub.2 present is undesirably long.
Accordingly, it would be advantageous if a procedure could be devised wherein SO.sub.2 breakthrough from the catalytic reduction step could be measured directly and rapidly with simplistic analytical equipment. Incorporation of such a procedure for monitoring SO.sub.2 coupled with an appropriate control system on the feed streams into the hereinbefore described processes for reducing the total sulfur content of Claus off-gases would substantially reduce the problems associated with carry through of SO.sub.2 from the catalytic reduction step into subsequent stages of the processes.